There are many thousands of apps available. But some apps are simply must-haves--whether for their functionality, interface brilliance, or sheer entertainment factor.

There are many thousands of apps available. (As Apple’s advertising campaign says, “There’s an app for that.”) But some apps are simply must-haves—whether for their functionality, interface brilliance, or sheer entertainment factor.

With the third-generation iPad now on retail shelves, chances are there are a lot of newly minted iPad owners out there wondering which apps to download first. Here are my must-haves. Note that I left out Apple’s own iOS offerings, though many—including the newly updated GarageBand, iMovie, and iWork suite and the just-released mobile version of iPhoto—are tremendously impressive and worth a download. (And if you’re looking for even more download ideas, a few months ago, my colleagues came up with a list of 50 essential iOS apps, which include more than a few dandy iPad-optimized offerings.)

Instapaper

Once you start relying on Instapaper (£2.99), you’ll wonder what took you so long. The idea is simple: Reading on your iPad is more pleasant than reading on your Mac’s screen.

Save lengthy articles for distraction-free reading later with Instapaper.

When you come across lengthier articles online, you tap the Instapaper button—whether in your browser, RSS reader, or Twitter client, or in any of the many other apps that integrate with the Instapaper service. The next time you launch Instapaper on your iPad, it will pull down the text of that article, and any inline images, too—but it’ll leave all the navigation, social networking modules, and Flash advertisements by the wayside. You’re left with just text and images, and you control the font and brightness and all that other good stuff. The app also makes it easy to discover other good Web content to read, based on your friends’ suggestions.

Tweetbot

Tweetbot began life as an excellent Twitter client for the iPhone, and the iPad version is even better. The £1.99 app’s unique interface and brilliantly implemented gesture support make it not just a powerful app for reading and posting tweets, but a fun one, too.

Everything you could want out of twitter is available at your fingertips with Tweetbot.

Swipe to the right on a tweet to see the full conversation surrounding it; swipe to the left to see replies sent to it. Tap and hold—on a tweet, a hashtag, a username, or a link—to expose contextual options related to that element. With support for services like Tweet Marker (for keeping you in sync with your device or desktop Twitter client), Instapaper (see above), and more, it’s a full-featured Twitter app that’s a delight to use.

Netflix

It feels almost like science fiction when you first use the Netflix app to stream movies and television shows to your iPad.

If you have a Netflix subscription, you want this app.

You can browse your Watch Instantly queue, search for other titles, and begin playing any of them in seconds. Netflix isn’t the iPad’s most elegantly implemented app; it feels a bit like a website crammed into a program. But it does what it’s supposed to do, which is to let you stream movies! Over the Internet! Wherever you have a reasonable Internet connection. And it’s a free download, to boot.

Flipboard

Flipboard takes content you’re interested in and presents it in an impressive magazine-inspired layout. The free app connects to your Facebook, Twitter, and Google Reader feeds, and also offers a variety of curated feeds in various categories like politics, technology, and entertainment.

Browse articles from around the web with Flipboard’s magazine-like interface.

Whether you’re browsing stories from social networks or the curated feeds, Flipboard jettisons photos wherever it can, and makes it easy to swipe through story after story. Andof course,the app offers easy ways to share interesting articles via Twitter, Facebook, and email, and to save them to Instapaper.

PCalc Lite Calculator

If you want a calculator on your iPad, PCalc is an excellent choice.

Though the iPhone’s Calculator app works fine, such an app simply doesn’t exist on the iPad. The free PCalc Lite works on all iOS devices, and it looks great on the iPad.

Beyond that, it adds tons of functionality beyond simple arithmetic: a scientific calculator, unit conversions, constants, Reverse Polish notation, multiple undo and redo, and themes. A £6.99 version comes packed with features, but if you start with the free Lite edition, you can add other options from the paid version with separate in-app purchases.

Dropbox

Dropbox, a free Web service, lets you create a folder on your Mac that syncs automatically with whatever other computers you tell it to.

There’s no easier way to access documents from your Mac on your iPad than with Dropbox.

The free iPad app isn’t beautiful, but it does afford you access to all the files and folders you store in your computer’s Dropbox folder. You can upload your saved photos and videos to your Dropbox folder, or open saved files in compatible apps on your iPad—including word processing documents, PDFs, images, and MP3s.

Toontastic

If you have kids, do them—and yourself—a favor by downloading a free copy of Toontastic. The app empowers kids to create their very own cartoons, walking them through the process of picking out scenery and characters (or drawing their own), adding built-in background music, and recording narration.

Your kids will delight in creating full-featured cartoons with Toontastic, and you’ll no doubt delight in watching them.

You can save and rewatch your kids’ Spielberg-quality creations, and optionally share them online, too. The app is adorably designed, and simple enough for the typical four-year-old to master.